1. Field of the Invention
The present invention relates to a digital-to-analog converter, and more particularly, to a digital-to-analog converter capable of fast switching of a digital input signal to an analog output voltage, and of precise voltage conversion.
2. Description of the Related Art
Digital-to-analog converters are used to convert a digital input signal to an analog output signal. The simplest technique used in a digital-to-analog converter is to use serially-connected resistors for dividing a voltage into multiple stages, between two power supply voltages. One of the digital-to-analog converters using this technique is disclosed in U.S. Pat. No. 5,831,566.
In U.S. Pat. No. 5,831,566, a plurality of resistors connected to each other in series are provided, and a plurality of switches, which are controlled by the output of a decoder for decoding a digital input signal, connect nodes between resistors to a power supply voltage VDD or a ground voltage VSS. Consequently, an analog output voltage is generated and output to the node of a resistor fixed to an analog output port. However, in U.S. Pat. No. 5,831,566, serial connection of resistors for dividing a voltage and outputting the divided voltages to an analog output port increases the resistance of the analog output port, so that switching to an analog output voltage is inevitably somewhat slow.
Digital-to-analog converters which can rapidly switch a digital input signal into an analog output voltage are desirable.
An object of the present invention is to provide a digital-to-analog converter which can rapidly switch a digital input signal into an analog output voltage.
To achieve the above object, an embodiment of the present invention provides a digital-to-analog converter (DAC) for voltage-converting a digital input signal into an analog output voltage. The DAC of the invention includes an auxiliary converter for determining the range of the analog output voltage by controlling connection to a first or second reference voltage in response to a range control signal. The DAC also includes a main convener for changing the analog output voltage by controlling connection to the first or second reference voltage in response to the digital input signal for directing voltage conversion.
In one embodiment, in the DAC of the invention, the auxiliary converter includes a resistor connected to the analog output voltage. A switch unit determines whether the resistor is to be connected to the first reference voltage or the second reference voltage, in response to the range control signal.
The main converter can include a decoder which decodes the digital input signal to direct voltage conversion. In one embodiment, the decoder is a binary-to-thermometer decoder in which the output code has a minimum code change. A resistor array of the main converter includes a plurality of resistors arranged in parallel, each of which is connected at a first end to the analog output voltage. A switch unit determines whether the other end of each resistor is to be connected to the first reference voltage or the second reference voltage in response to an in output code tot the decoder. In one embodiment, each of the resistors in the array has the same resistance. In an alternative embodiment, the resistors have binary-weighted resistances. In this embodiment, the on resistance of the switch unit is reduced according to a binary-weighted ratio of the resistors in the array.
In another aspect, the present invention provides a DAC for converting the voltage of a digital input signal into an analog output voltage. The DAC of this aspect includes an auxiliary converter for determining the range of the analog output voltage by controlling connection to a first or second reference voltage in response to a range control signal. To direct signal conversion, a main converter divides the digital input signal into upper bit digital input signals and lower bit digital input signals. The main converter changes the analog output voltage in response to the lower bit digital input signal with a first resistor array having a resistance that is a binary-weighted unit resistance, and changes the analog output voltage in. response to the upper bit digital input signal with a second resistor array having the same unit resistances.
In one embodiment of this aspect of the invention, in the main converter, the resistors in the first resistor array are coupled to the first or second reference voltage in direct response to the lower bit digital input signal, and the resistors in the second resistor array are coupled to the first or second reference voltage in response to the output code of a decoder which responds to the upper bit digital input signal. In another embodiment of this aspect, the resistors in the first resistor array are coupled to the first or second reference voltage in response to the output code of a decoder which responds to the lower bit digital input signal, and the resistors in the second resistor array are coupled to the first or second reference voltage in response to the output code of a decoder which responds to the upper bit digital input signal. In either of these particular embodiments, the decoder can be a binary-to-thermometer decoder n which the output code has a minimum code change.
In still another aspect, the present invention provides a DAC for converting the voltage of a digital input signal into an analog output voltage. The DAC of this aspect includes an auxiliary converter for determining the output voltage range of the analog output port of the DAC by controlling connection to a first or second reference voltage in response to a range control signal. To direct voltage conversion, a main converter divides the digital input signals into row-direction bit digital input signals and column-direction bit digital input signals. The main converter changes the analog output voltage by resistor-switching cells which respond to the output code of a column decoder to decode the column-direction bit digital input signal and the output code of a row decoder to decode the row -direction bit digital input signal.
In one embodiment of this aspect of the invention, each of the column decoder and the row decoder is a binary-to-thermometer decoder in which the output code has a minimum code change.
Each of the resistor-switching cells can include a local decoder responsive to the output codes of the column decoder and row decoder. A resistor is coupled to the analog output voltage, and a cell switch unit connects the resistor to the first or second reference voltage in response to the output of the localdecoder.
Each of the resistor-switching cells can have a matrix structure in which the cells are arranged in rows and columns in accordance with the number of output codes of the row decoder and the number of output codes of the column decoder.
Such a digital-to-analog converter according to the present invention can achieve fast switching and precise conversion of a digital input signal into an analog output voltage, and diversify the range of an analog output voltage.